As electric equipments are becoming smaller and smaller, a stepping motor of claw pole type (so-called “PM stepping motor”) for use in such electric equipment is requested to be downsized and also to provide an enhanced performance. Under such circumstances, a rotor magnet must be downsized and also improvement must be sought in the method of fixing a rotor shaft to the magnet.
For example, an Nd—Fe—B bonded magnet having a high magnetic characteristic is commonly used as a rotor magnet, and such a magnet is fixed to a rotor shaft such that an adhesive is applied to a portion of the outer circumference of the shaft, and then the shaft is inserted through an axial hole of the magnet thereby filling the adhesive into a gap present between the shaft and the magnet.
In this method, the shaft with adhesive applied thereon is slid through the hole of the magnet to its predetermined position, and some portion of the adhesive is not introduced into the hole of the magnet, specifically into the gap between the shaft and the magnet, and is caused to build up on the shaft at one end of the magnet, and it is necessary to wipe the built-up adhesive off the shaft. Also, since even a portion of the adhesive that is required to duly fix the shaft to the magnet is not introduced into the gap between the shaft and the magnet, it happens that the shaft can not be kept securely fixed to the magnet.
In order to overcome the above problem of adhesive building up at a shaft at one end of a magnet, a truncated cone-shaped recess as an adhesive reservoir is formed at one end of the magnet (refer, for example, to Japanese Utility Model Application Laid-Open No. H04-097445).
FIG. 1A shows a side of such a rotor as described above, wherein a shaft 102 is to be inserted in a hole of a magnet 101, and FIG. 1B shows a cross section of the magnet 101 shown in FIG. 1A.
Referring to FIG. 1A, an adhesive 105 is applied to a portion of the shaft 102 in a line (or at a point), and the shaft 102 with the adhesive 105 applied thereon is inserted into a hole 103 of the magnet 101 from an end of the magnet 101 provided with a truncated cone-shaped recess 104. Thanks to the recess 104 having a truncated cone shape with an inclined wall, the adhesive 105 is adapted to be smoothly introduced into a gap between the shaft 102 and the inner circumferential surface of the hole 103 and well spread in the gap, thus enhancing the pull-out resistance of the shaft 102 with respect to the magnet 101. After the shaft 102 is inserted and positioned in place, some portion of the adhesive 105 that is not introduced into the gap is lodged in the recess 104 and does not protrude from the end of the magnet 101 thus not obstructing the rotation of the rotor.
In this method, however, due to the adhesive 105 filled in the gap between the inner circumferential surface of the hole 103 and the outer circumferential surface of the shaft 102, it may happen that the magnet 101 and the shaft 102 are not disposed coaxial with each other, in which case the rotor has a runout thereby causing vibration. In order to deal with this runout problem, the magnet 101 and the shaft 102 must be supported by a tool coaxially with each other until the adhesive 105 is cured, which results in a deteriorated workability.
Under the circumstances, a rotor is disclosed in which a shaft is fixedly disposed coaxially with a magnet in a mechanical manner (refer, for example, to Japanese Patent Application Laid-Open No. H09-200983).
FIG. 2 shows a side (partly cross-sectioned) of a rotor as described above, and FIG. 3 shows an axial end of a magnet 210 shown in FIG. 2.
Referring to FIG. 3, the magnet 210 has a hole 211 and includes a shaft retaining mechanism structured such that a plurality (four in the figure) of retainer segments 214 each defining a flat face 217 are provided equiangularly at an inner circumferential surface 211a of the magnet 210, wherein a circular line connecting the center points of respective flat faces 217 of the retainer segments 214 defines a virtual precise circle which is situated in a concentric manner with the magnet 210. The retainer segments 214 extend axially to divide the inner circumferential surface 211a into a plurality (four in the figure) of curved segments 216.
Referring to FIGS. 2 and 3, a shaft 220 is inserted in the hole 211 of the magnet 210 and supported therein by the retainer segments 214 of the shaft retaining mechanism, specifically at the center points of the flat faces 217 of the retainer segments 214, and adhesive is filled in gaps formed between the shaft 220 and the curved segments 216, whereby the shaft 220 can be securely fixed to the rotor magnet 210 and at the same time can be set coaxially with respect to the magnet 210.
In the structure of the rotor described above, however, the gap between the shaft 220 and the curved segments 216 is small, and consequently it is difficult to fill adhesive into the gap. Also, it is difficult to know the amount of adhesive which is introduced in the gap between the shaft 220 and the curved segments 216.